Hydrocarbons sometimes exist in a formation but cannot flow readily into the well because the formation has very low permeability. In order for hydrocarbons to travel from the formation to the wellbore there must be a flow path from the formation to the wellbore. This flow path is through the formation rock and has pores of sufficient size and number to allow a conduit for the hydrocarbons to move through the formation. In some subterranean formations containing hydrocarbons, the flow paths are of low incidence or occurrence and/or size that efficient hydrocarbon recovery is hampered.
With respect to wells that previously produced satisfactorily, a common reason for a decline in oil and gas production from a particular formation is damage to the formation that plugs the rock pores and impedes the flow of oil to the wellbore and ultimately to the surface.
Well stimulation refers to the various techniques employed to improve the permeability of a hydrocarbon-bearing formation. Three general well-stimulation techniques are typically employed. The first involves injecting chemicals into the wellbore to react with and dissolve permeability damaging materials such as wellbore coatings, e.g. as may remain from previously used drilling fluids. A second method requires injecting chemicals through the wellbore and into the formation to react with and dissolve small portions of the formation thereby creating alternative flow paths for the hydrocarbons to flow to the wellbore. These alternative flow paths redirect the flow of hydrocarbons around the low permeability or damaged areas of the formation. A third technique, often referred to as fracturing, involves injecting chemicals into the formation at pressures sufficient to actually fracture the formation, thereby creating a relatively large flow channel though which hydrocarbon can more readily move from the formation and into the wellbore.
Acidizing treatments of wells are a conventional process for increasing or restoring the permeability of subterranean formations so as to facilitate the flow of oil and gas from the formation into the well. The acid treatment is to remove formation damage along as much of the hydrocarbon flow path as possible, and/or to create new flow paths. An effective treatment should remove as much damage as possible along the entire flow path. This process involves treating the formation with an acid to dissolve fines and carbonate scale plugging or clogging the pores, thereby opening the pores and other flow channels and increasing the permeability of the formation. Continued pumping forces the acid into the formation, where it etches channels or wormholes. These channels provide ways for the formation hydrocarbons to enter the well bore.
More recently, multi stage fracture (MSF) systems have been used in several horizontal boreholes as another form of well stimulation technique. These systems provide a unique way to create compartments along the well, where packers, mechanical or swellable, may provide a hydraulic seal to stimulate that particular section of the well. By stimulating each compartment individually, the stimulated contact area between the formation and well increases, thus positively affecting the hydrocarbon production performance. However, in such cases hydraulic isolation between compartments is compromised due chemical reaction between the stimulation fluid and rock or reservoir behind the packer elements. MSF systems may use this acid to stimulate the carbonate formation, and may be normally orientated to breakdown the formation and create hydraulic fractures in the formation by using a combination of reactive and non-reactive fluids. Conventional acidizing fluids, such as hydrochloric acid or a mixture of hydrofluoric and hydrochloric acids, have high acid strength and quick reaction with fines and scale nearest the well bore. These particular acids, however, have a tendency to corrode tubing, casing and down hole equipment, such as gravel pack screens and down hole pumps, especially at elevated temperatures, in particular, above 200° F. (92° C.). In many cases organic acids are used in place of hydrochloric acid, such as formic and acetic acids. However, even if the packer elements and fracture port are placed in the right places, the reactive fluid movement near the packer elements can be minimal and depends just on the reservoir injectivity or mobility, which may be dictated by the permeability of the formation and the reservoir pressure.